1. Field of the Invention
The invention relates to electrospray ionization. In particular, the invention relates to devices used for achieving a microfluidic sample stream which can be broken into droplets, ionized and sprayed, for example, for the purposes of mass spectrometry. The invention concerns also a method of manufacturing an electrospray device of the present kind and a method for performing mass-spectrometric analyses.
2. Related Art
Electrospray ionization (hereinafter also abbreviated as “ESI”) is a technique used in mass spectrometry to produce ions. In conventional electrospray ionization, a liquid is pushed through a very small charged capillary. This liquid contains the analyte to be studied dissolved in a large amount of solvent, which is usually more volatile than the analyte. In ESI, the analyte is typically dissolved in a polar solvent, for example methanol, and is introduced into a mass spectrometer through a thin needle-shaped capillary tube. When the capillary is exposed to high voltage (2-5 kV), a strong electrostatic field is formed at the tip of the capillary and, as a result, a charged aerosol is formed in the gaseous phase from the solution coming out of the capillary. The charged droplets of the aerosol emit gaseous-phase ions into the gaseous phase. The ions are collected into a mass analyser of a mass spectrometer.
Mass spectrometry is used in many fields of science, such as pharmaceutical research, life sciences, and food and environmental analysis. In mass spectrometry (hereinafter also abbreviated as “MS”) material is examined on the basis of data about its mass, and with MS it is possible, among other things, to identify the compounds of a chemical sample and to determine their quantity (<10−11 M) in very low concentrations, from complex sample matrices. In ESI-MS gas-phase ions is generated as described above and the ions are separated on the basis of their mass/charge ratio (m/z) using electric and/or magnetic fields (mass analyser). The gas-phase ions are observed using a detector. The spectrum of the mass is established from a graph of the strength of the ionic current, which is generated by the detector, as a function of the m/z value of the ion. ESI is suitable for examining even large molecules (MW>100 kDa).
The current trend in analytical chemistry during recent years has been the miniaturization of analytical devices, using microfabrication technology. The goal is to integrate different miniaturized components on a lab-on-a-chip device, allowing faster and cheaper analyses with smaller amounts of sample than with conventional analytical devices. The common means of transferring liquids in microchannels of lab-on-a-chip devices are electroosmosis or pressure-driven flows. The drawback with both of these techniques is that an additional device, such as a pump or a high-voltage supply, is needed.
Miniaturized ESI solutions are already known, where flow channels for the sample solution and an injection tip used for ionising are machined in a monolithic, small glass plate, for example. Hereinafter, these devices are also called “ESI micro chips” or “μESI devices”. Early developments of this kind of technology are described in U.S. Pat. Nos. 6,481,648 and 6,245,227.
Of more recent publications relating to ESI technology, US 2002/0139751 is mentioned. The device disclosed in the publication comprises a chip having a channel fabricated through a silicon wafer and extending from the tip of the chip to containers manufactured on the other side of the chip. JP 2005/134168, WO 2007/092227 and WO 2006/049333 disclose ESI devices comprising hollow channels, which are filled with porous material. US 2005/0116163 discloses an ESI needle comprising a channel, which may have a twisted or wavy inner geometry. JP 2005/190767 discloses an ESI nozzle made from metal-coated glass. Wire material may be included in the nozzle for aiding sample transfer. U.S. Pat. No. 6,297,499 discloses an ESI device, wherein the sample is conveyed to the spraying region using wicks. US 2002/0000507 discloses an electrospray device comprising a silicon substrate having a through-fabricated channel and an injection zone on the other surface of the substrate. The devices referred to above basically require pumping for sample transfer or high sample flow rates, or are prone to clogging.
Several other microchip based electrospray tips have also been developed during last few years as shown in recent scientific reviews by Lazar et al (I. Lazar, J. Grym, F. Foret, Mass Spectrom. Rev. 2006, 25, 573-594) and Sung et al (W-C. Sung, H. Makamba, S-H. Chen, Electrophoresis 2005, 26, 1783-1791). Shortly, these electrospray tips are made of either glass or polymers such as PDMS (polydimethylsiloxane), PMMA (polymethyl methacrylate) or SU-8, and they are based on off-chip spraying microdevices, in which a ESI capillary is separately attached to a microchip or on on-chip spraying microdevices, where the ESI tip is an integral part of a microchip. In these ESI microchips the liquid flow is generated either by means of pumps or electroosmosis.
Brinkmann et al (M. Brinkmann, R. Blossey, S. Arscott, C. Druon, P. Tabourier, S. Le Gac, C. Rolando, Appl. Phys. Lett. 2004, 85, 2140-2142) and Arscott and Troadec (S. Arscott, D. Troadec, A nanofluidic emitter tip obtained by focused ion beam nanofabrication, Nanotechnology 16 (2005) 2295-2302) have utilized capillary forces in a rectangular capillary slot for liquid transport from a reservoir to a cantilever ESI tip made from SU-8 and polycrystalline silicon (polysilicon). In addition to in-plane tips there are also silicon ESI tips with out-of-plane design (W. Deng et al./Aerosol Science 2006, 37, 696-714 and S. Zhang, C. K. van Pelt, J. D. Henion, Electrophoresis 2003, 24, 3620-3632).
The most popular fabrication materials of ESI chips have been glass (Q. Xue, F. Foret, Y. M. Dunayevskiy, P. M. Zavracky, N. E. McGruer, B. L. Karger, Multichannel microchip electrospray mass spectrometry, Anal. Chem. 69 (1997) 426-430 and R. S. Ramsey, J. M. Ramsey, Generating electrospray from microchip devices using electroosmotic pumping, Anal. Chem. 69 (1997) 1174-1178) and polymers, such as parylene (X.-Q. Wang, A. Desai, Y.-C. Tai, L. Licklider, T. D. Lee, Polymer-based electrospray chips for mass spectrometry, Tech. Digest, IEEE MEMS, Orlando, 1999 pp. 523-528), PDMS (H. Chiou, G.-B. Lee, H.-T. Hsu, P.-W. Chen, P.-C., Liao, Micro devices integrated with channels and electrospray nozzles using PDMS casting techniques, Sens. Actuators, B, Chem. 86 (2002) 1-7), and SU-8 (S. Tuomikoski, T. Sikanen, R. A. Ketola, R. Kostiainen, T. Kotiaho, S. Franssila, Fabrication of enclosed SU-8 tips for electrospray ionization-mass spectrometry, Electrophoresis 26 (2005) 4691-4702). However, these materials set limits to chip designs.
Silicon ESI chips (A. Desai, Y.-C. Tai, M. T. Davis, T. D. Lee, A MEMS electrospray nozzle for mass spectrometry”, Tech. Digest, IEEE Transducers, Chicago, 1997, pp. 927-930 and S. Zhang, C. K. Van Pelt, J. D. Henion, Automated chip-based nanoelectrospray-mass spectrometry for rapid identification of proteins separated by two-dimensional gel electrophoresis, Electrophoreses 24 (2003) 3620-3632) have also been realized because of the well-explored microfabrication techniques of silicon. However, the conductivity of the silicon limits its use, because it excludes the use of electroosmotic flow in sample transport. Pressure driven flow has been the other popular method used for sample transportation in previous ESI chips. However, both of these methods require an external actuator, such as a high-voltage supply or a pump. Pressure driven flows also require the use of troublesome fluidic connectors. Some ESI chips exploit capillary forces to transport the sample, but narrow or closed channels are usually required in order to achieve sufficiently strong capillarity.
Despite recent developments in this field, there is still a constant demand for faster, easier-to-use, more selective, more sensitive and reliable analysis devices and methods especially for drugs and biomolecules using smaller sample volumes.